The gravitational lensing effect can be used to spot black holes in another way.
If a black hole passes between us and a single distant star, the black hole would focus the light from the star into our telescope and, instead of causing it to blink out by passing in front of it, it would instead cause the star to appear temporarily brighter.
Thus another way of looking for black holes is to observe distant stars, perhaps by computer, and look for this characteristic temporary brightening. This slide shows a candidate event of such a "micro-lensing effect". The star in the box in the lower left picture appears brighter in November, 1996 than it did in April, 1996 because there is an invisible, dark very heavy object directly between the star and us, focussing the light into our telescope. Unfortunatly, this particular event seems to be due to a compact burnt out star, rather than a black hole, but the search continues...